Abstract A number of gaps remain in our understanding of the effects of modifiable and non-modifiable factors on bone development during childhood and adolescence, and the impact of early bone health on the risk for common adult conditions such as osteoporosis and bone fractures. Few pediatric studies have longitudinally assessed bone accrual and changes in bone geometry. Even for physical activity and well-known dietary factors such as fluoride, calcium, and vitamin D, no large cohort of children has been followed to describe bone development from birth through later adolescence. The Iowa Bone Development Study (IBDS), in its 11th year, is a longitudinal cohort investigation of the effects of behaviors, selected physical attributes, and genetic factors on bone development. Our goal is to increase the understanding of childhood/adolescent bone development and determinants of bone health by extending our assessments into later adolescence (ages 15 and 17), thus allowing a comprehensive assessment of bone development from age 5. It has been estimated that by age 17, approximately 86 and 93% of total peak bone mass has accrued for boys and girls, respectively. We have assessed bone measures using dual energy x-ray absorptiometry (DXA) and peripheral quantitative computed tomography (pQCT) for our cohort at 5 (n=470), 8 (n=539), and 11 (n=483) years of age; age 13 assessments are nearly complete (n=466 to date); and age 15 exams have been initiated (n=111). Fluoride intake and dietary data have been collected from birth; physical activity assessment continues by accelerometry and questionnaire; and candidate genes involved in bone mineralization, matrix structure, and growth have been studied. We also have analogous explanatory and bone outcome measures for parents of cohort members. In this renewal, we will collect age 15 and 17 bone, fluoride intake and dietary, alcohol, tobacco, physical activity, maturity, and body composition data. We propose to: 1) Assess and define the time-specific and cumulative effects of fluoride intake on bone mass and geometry at age 17 and bone development longitudinally from age 5 to age 17. 2) Assess and define the time-specific and cumulative effects of behaviors (dietary patterns, alcohol intake, tobacco use, and physical activity) and physical attributes (gender, growth and development, body composition, and muscle strength) on bone mass and geometry at age 17 and bone development longitudinally from age 5 to age 17. and 3) Assess and define the time-specific and cumulative effects of behaviors, physical attributes, and genetic factors, as well as interaction effects (environment x environment gene environment, and gene gene), on bone mass and geometry at age 17 and bone development longitudinally from age 5 to age 17, building on the results obtained by addressing aims #1 and 2 above. Continued follow-up of this cohort will lead to major insights into the relative importance of fluoride and these other factors on bone development and the times of their greatest impact, thus improving our understanding of the best strategies to optimize skeletal health.